A photosite comprises at least one photodiode and at least one isolation trench zone for isolating the photodiode or photodiodes.
FIG. 1 illustrates a front-side illumination imaging device such as, for example, that described in the article by Matsunaga, et al., Electron Devices, IEEE Transactions, May 1991, 38 (5), pp. 1044-1047 and the article by Yamashita, et al., Electron Devices Meeting, 1988. IEDM '88. Technical Digest, International, 1988, pp. 78-81. This device is a front-side illumination device, which means a device in which the photons pass through the interconnection part (including especially metal tracks or metal levels, separated by dielectric layers and interconnected, in some cases, by vias between the metal levels), which is located above the active components, and especially the read transistor, before arriving on the photodiode.
This interconnection part is commonly denoted by those skilled in the art by the term BEOL (Back End of Line). The device (see FIG. 1) comprises an n-type doped substrate 1 in which a p-doped zone 2 is produced, for example by implantation. Produced above such a structure is an active zone 3, within which the dissociation of the charges takes place by photoelectric effect, and two P+-doped zones 4, 5 located on either side of the active zone, so as to form the source and drain regions of a transistor that has here two superposed gates 6, 7.
In operation, the charges are accumulated in the active zone 3 during a cycle. The charges are accumulated because the p-doped zone 2 forms a potential barrier preventing the charges from rejoining the substrate 1. The accumulated charges are measured by integration by the transistor.
At the start of a new cycle, the accumulated photogenerated charge must be dissipated. To do so, a potential pulse is applied to the gate 6 so as to suppress the potential well in active zone 3. With the potential well suppressed, the photogenerated charges can pass through the zone 2 so as to be discharged into the substrate 1 acting as a draining zone.
The current trend in imaging devices is towards back-side illumination devices because of their high quantum efficiency. The quantum efficiency is the ratio of the number of photogenerated charges to the number of incident photons.
A back-side illumination imaging device is an imaging device in which the photons arrive directly on the photodiode, the interconnection part being produced on that side of the photodiode opposite the side via which the photons arrive, the substrate having been removed during fabrication of the component.
However, the teaching of the prior art is not compatible with such back-side illumination imaging devices. This is because, in the case of a back-side illumination photodiode configuration, the substrate is no longer present and therefore cannot be used as a draining zone.